Liquid crystal display (LCD) panels do not emit light spontaneously and hence usually achieve the display function via a backlight module in order for users to clearly see contents displayed by the LCD panels. The backlight module comprises light sources, an optical membrane, a reflector, a back plate and other necessary optical and structural components, and can be divided into an edge-lit type and a direct-lit type.
The light sources (such as light-emitting diode (LED)) of an edge-lit backlight are disposed aside a panel. Light emitted by the LED is directly incident into a light guide plate from one side and transmitted to the other end from this side of the light guide plate. The light is diffused to various directions when encountering diffusion points, and then emitted out through a light-emitting surface of the light guide plate. The main function of the light guide plate is to convert incident parallel light into planar light. Subsequently, the light emitted by the light guide plate is diffused and deflected by diffusion. Finally, the light converge within a same surface direction via two condensing prism sheets of which the surfaces take the shape of continuously jagged grooves, and the light divergence angle is adjusted.
In a direct-lit backlight structure, light sources are disposed under (or behind) an LCD panel. Light emitted by light sources on light bars is mixed during travel, diffused by a diffusion plate, and finally emitted out as a surface source.
With the development of the LED packaging technology, large-size LED LCD TVs (LCD TVs using LED for backlight) have entered the market and soon dominate the market. However, due to the limitations of brightness, efficiency and the like of LEDs, large-scale edge-lit backlights cannot meet the requirements of high brightness and high optical uniformity of a display device at present. Currently, all the LED LCD TVs with the large size of more than 55 inches adopt direct-lit backlight modules. The traditional direct-lit backlight modules mostly adopt small-size LEDs as light sources of the backlight modules, and the light-emitting surface of each LED is directly opposite to a diffusion plate. Due to the high concentration and strong light converging capability of the LEDs, spots of different brightness tend to be formed on the surface of the backlight module. In order to obtain a surface source with good uniformity, a plurality of LEDs are required to be arranged or the light mixing distance from the LED light-emitting surface to the diffusion plate is required to be increased, and hence the manufacturing cost of the backlight module is increased and the development of the backlight module towards low weight and thin profile is limited.